How would it be possible to get the file path of a file (for example a Classic Nisus Writer file, or a .doc file) which is "temporarily" open as "xxx (converted)".

In fact, I already asked this question to the Nisus mailing list, and I was answered that there is no command for that, and there is no temporary file created for the converted document (so all is done in RAM).

In that case, I think this is impossible for now; but there may be situations in which such an information is needed (for example, I want to save the content of a converted file as a Unicode text file). So this is rather a feature request now.